Magnetic coils are used to apply non-invasive electromagnetic deep nerve stimulation therapy. The stimulation therapy is applied using a magnetic coil constructed of electrically conducting material, such as copper wire, copper tubing, or Litz wire. Typically, these coils can have an inductance in the range of a few μH up to 20 μH, and may or may not be cooled to avoid overheating. The coils are excited with time varying current pulses, in the form of single pulses, pulse trains or other periodic waveforms, in order to produce a corresponding time-varying magnetic field that penetrates the biological tissues. The magnetic field induces an electrical field in the tissues, which is shaped by the electrical conductivities of different tissues and anatomical structures and reaches the nerves buried deep within those structures.
The induced electrical field acting on deep nerve neurons can cause depolarization of the membrane potential of the neuron. If this depolarization reaches a threshold level, the neuron will fire an action potential, or impulse. Whether an electric field stimulates a nerve to a degree sufficient to trigger an action potential depends on a variety of parameters, such as the strength, gradient, duration, and direction of the electric field at the target nerve, as well as the anatomy of the nerve itself. While the nerve anatomy is fixed and cannot be controlled the electric field acting on the nerve can be manipulated.
The general term “field” can refer to either or both of a magnetic field and an electric field, whichever is applicable. Generally speaking, in terms of neural stimulation, the field that produces the stimulus is the electric field acting on the neural structure. Traditionally, to focus an electric field on a specific target, such as in the field of Transcranial Magnetic Stimulation (TMS), a double coil, normally referred to as a figure eight coil or a butterfly coil is used, where the maximum electric field falls along a perpendicular axis from the meeting point of two equal diameter coils.
In order to stimulate deeper targets, one simple solution is to increase the diameter of the two coils. However, increasing the diameter of the coils has the effect of reducing the coil focality. While there have been many attempts to overcome this depth-focality trade-off, the consensus in the field has been that, for traditional coil designs, the ability to stimulate deeper targets is obtained at the expense of a reduction in the coil focality. As a consequence, target depths greater than 4 cm have been considered to be unsafe for the purposes of brain stimulation.
Non-invasive magnetic stimulation of nerve structures deep in the head has received a great deal of attention in the neurological sciences. However, attempts to stimulate structures deep in the brain do so with the intent being to produce a narrow, needle-like field distribution so as to avoid the off-target neural structures or regions (“off-targets”) while stimulating the target neural structures or regions (“targets”).
Additionally, generating these electric fields can create a great deal of heat that, owing to the fact that the stimulator necessarily is placed in close vicinity to the patient, creates a need for effective cooling. Deep nerve stimulators can be high voltage devices (e.g., 2000 volts) and can generate large electric fields for extended durations. For example, a deep nerve stimulator can be asked to deliver 300 millisecond, 2 Tesla pulses for 5 minutes. Absent effective cooling, coils delivering this level of stimulation could reach skin burn temperature of 42 degrees C. in 10 seconds and could fail due to overheating in 25 seconds.